For many molding applications, polyethylene terephthalate crystallizes much too rapidly. Although the undesirably high crystallization tendency of polyethylene terephthalate can be reduced by replacing part of the terephthalic acid or the glycol by compounds that by their constitution reduce the tendency of the resin to crystallize, most of such compounds have an adverse effect on the desirable properties of the polymer such as mechanical strength, solvent resistance, and abrasion resistance. Furthermore, such compounds generally reduce the glass temperature of the resin, thereby reducing the utility of the polymer. For example, the replacement of only 10 mole percent of the terephthalic acid in the polymer with glutaric acid drops the glass temperature from 73.degree. to 59.degree. C.
In contrast, the substitution of only 7 mole percent tetrachlorobenzenedimethanol for the ethylene glycol in polyethylene terephthalate actually raises the glass temperature from 73.degree. to 83.degree. and at the same time greatly reduces the tendency of the polymer to crystallize. (In this specification tetrachlorobenzenedimethanol is abbreviated TCBDM.) Furthermore, if the copolyester is made to contain only slightly more than 15 mole percent TCBDM the glass temperaof the resulting copolymer is well above 100.degree. C. and now there is almost no tendency of the polymer to crystallizing. Thus, products made from the copolymer will not deform in boiling water, and indeed, are even steam sterilizable. It is obvious that such copolymers having glass temperatures above 100.degree. C. will have a much greater range of applications than polyethylene terephthalate with its glass temperature of only 73.degree. C. The desirable glass temperatures of these copolymers also points to uses as a high strength substrate for other polymeric films or coatings which have lower glass temperatures.